Field of the Invention
This invention relates to seismology and more particularly relates to the deployment and retrieval of seismic autonomous underwater vehicles by an underwater vehicle or surface vehicle.
Description of the Related Art
Marine seismic data acquisition and processing generates a profile (image) of a geophysical structure under the seafloor. Reflection seismology is a method of geophysical exploration to determine the properties of the Earth's subsurface, which is especially helpful in determining an accurate location of oil and gas reservoirs or any targeted features. Marine reflection seismology is based on using a controlled source of energy (typically acoustic energy) that sends the energy through seawater and subsurface geologic formations. The transmitted acoustic energy propagates downwardly through the subsurface as acoustic waves, also referred to as seismic waves or signals. By measuring the time it takes for the reflections or refractions to come back to seismic receivers (also known as seismic data recorders or nodes), it is possible to evaluate the depth of features causing such reflections. These features may be associated with subterranean hydrocarbon deposits or other geological structures of interest.
In general, either ocean bottom cables (OBC) or ocean bottom nodes (OBN) are placed on the seabed. For OBC systems, a cable is placed on the seabed by a surface vessel and may include a large number of seismic sensors, typically connected every 25 or 50 meters into the cable. The cable provides support to the sensors, and acts as a transmission medium for power to the sensors and data received from the sensors. One such commercial system is offered by Sercel under the name SeaRay®. Regarding OBN systems, and as compared to seismic streamers and OBC systems, OBN systems have nodes that are discrete, autonomous units (no direct connection to other nodes or to the marine vessel) where data is stored and recorded during a seismic survey. One such OBN system is offered by the Applicant under the name Trilobit®. For OBN systems, seismic data recorders are placed directly on the ocean bottom by a variety of mechanisms, including by the use of one or more of Autonomous Underwater Vehicles (AUVs), Remotely Operated Vehicles (ROVs), by dropping or diving from a surface or subsurface vessel, or by attaching autonomous nodes to a cable that is deployed behind a marine vessel.
Autonomous ocean bottom nodes are independent seismometers, and in a typical application they are self-contained units comprising a housing, frame, skeleton, or shell that includes various internal components such as geophone and hydrophone sensors, a data recording unit, a reference clock for time synchronization, and a power source. The power sources are typically battery-powered, and in some instances the batteries are rechargeable. In operation, the nodes remain on the seafloor for an extended period of time. Once the data recorders are retrieved, the data is downloaded and batteries may be replaced or recharged in preparation of the next deployment. Various designs of ocean bottom autonomous nodes are well known in the art. Prior autonomous nodes include spherical shaped nodes, cylindrical shaped nodes, and disk shaped nodes. Other prior art systems include a deployment rope/cable with integral node casings or housings for receiving autonomous seismic nodes or data recorders. Some of these devices and related methods are described in more detail in the following patents, incorporated herein by reference: U.S. Pat. Nos. 6,024,344; 7,310,287; 7,675,821; 7,646,670; 7,883,292; 8,427,900; and 8,675,446.
Emerging technologies in marine seismic surveys need a fast and cost effective system for deploying and recovering seismic receivers that are configured to operate underwater. Newer technologies use AUVs that have a propulsion system and are programmed to move to desired positions and record seismic data. After recording the seismic data, the AUVs are instructed to return to a vessel or underwater base, such as shown in U.S. Publication No. 2014/0301161 and Publication No. WO2014/147165, incorporated herein by reference. Various systems and methods have been proposed for deploying, guiding, and collecting the AUVs. However, none of the existing methods fully address the needs of a seismic survey that uses deployable ocean bottom AUVs to collect the seismic data. For example, if AUVs are to be directly deployed and recovered from a surface vessel (besides having sophisticated equipment to allow such deployment and recovery), the AUV must have enough power to travel to the bottom of the seabed and after seismic data collection, resurface back to the vessel. This is very challenging, particularly in deep-water situations. Communications between each AUV and the surface vessel likewise encounter numerous difficulties.
A need exists for an improved method and system for deploying and retrieving AUVs on the ocean bottom, and in particular one that eliminates the requirement for each AUV to directly communicate with a surface vessel and also allows for faster deployment and recovery of the AUVs from a location other than a surface vessel.